A known radio frequency passive acoustic transponder system produces individualized responses to an interrogation signal. The code space for these devices may be, for example, 2.sup.16 codes, or more, allowing a large number of transponders to be produced without code reuse. These devices provide a piezoelectric substrate on which an aluminum pattern is formed, for example b a typical microphotolithography process, with a minimum feature size of, for example, one micron.
The known transponder devices include a surface acoustic wave device, in which an identification code is provided as a characteristic time-domain delay (reflection) pattern in a retransmitted signal, in a system which generally requires that the signal emitted from an exciting antenna be non-stationary with respect to a signal received from the tag. This ensures that the reflected signal pattern is distinguished from the emitted signal in a plurality of states, allowing analysis of the various delay components within the device. In such a device, received RF energy is transduced onto a piezoelectric substrate as an acoustic wave with a first interdigital electrode system, from whence it travels through the substrate, interfacing with reflector, delay or resonant/frequency selective elements in the path of the wave, and a portion of the acoustic wave is ultimately received by the interdigital electrode system and retransmitted. These devices do not require a semiconductor memory nor external electrical energy storage system, e.g., battery or capacitory, to operate. The propagation velocity of an acoustic wave in a surface acoustic wave device is slow as compared to the free space propagation velocity of a radio wave. Thus, the time for transmission between the radio frequency interrogation system and the transponder is typically short as compared to the acoustic delay, so that rate of the interrogation frequency change is based on the delay characteristics within the transponder. The interrogation frequency thus changes sufficient so that a return signal having a minimum delay may be distinguished from the interrogation frequency, and so that all of the relevant delays are unambiguously received for analysis. The interrogation frequency thus should not return to the same frequency before a maximum delay period. Generally, such systems are interrogated with a pulse transmitter or chirp frequency system.
Systems for interrogating a passive transponder employing acoustic wave devices, carrying amplitude and/or phase-encoded information are disclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160; 4,604,623; 4,605,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208; 4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790; 4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein by reference. Other passive interrogator label systems are disclosed in the U.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217.
In its simplest form, the acoustic transponder systems disclosed in these patents include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric "launch" transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of an electrical signal corresponding to the RF interrogation wave, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. This acoustic wave may be modified along its path, such as by reflection, attenuation, variable delay (phase shift), and interaction with other transducers or resonators.
When an acoustic wave pulse is reconverted into an electrical signal, it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. This energy is received at a receiver and decoder, typically at or near the same location as the interrogating transmitter, and the information contained in this response to an interrogation signal is decoded. The tag typically has but a signal antenna, used for both receiving the interrogation pulse and emitting an information bearing signal. Designs are known, however, with separate receiving and transmitting antennas, which may be at the same frequency or harmonically related, and having the same or different polarization.
In systems of this general type, the information code associated with and which identifies the passive transponder is built into the transponder at the time that a layer of metallization is finally defined on the substrate of piezoelectric material. This metallization thus defines the antenna coupling, launch transducers, acoustic pathways and information code elements, e.g., reflectors. Thus, the information code in this case is non-volatile and permanent. The information is present in the return signal as a set of characteristic perturbations of the interrogation signal, such as a specific complex delay pattern and attenuation characteristics. In the case of a transponder tag having launch transducers and a variable pattern of reflective elements, the number of possible codes is N.times.2.sup.M where N is the number of acoustic waves launched by the transducers and M is the number of reflective element positions for each transducer. Thus, with four launch transducers each emitting two acoustic waves, and a potential set of eight variable reflective elements in each acoustic path, the number of differently coded transducers is 2048. Therefore, for a large number of potential codes, it is necessary to provide a large number of launch transducers and/or a large number of reflective elements. However, efficiency is lost with increasing complexity, and a large number of distinct acoustic waves reduces the signal strength of the signal encoding the information in each. Therefore, the transponder design is a tradeoff between device codespace complexity and efficiency.
The transponder tag thus typically includes a multiplicity of "signal conditioning elements", i.e., delay elements, reflectors, and/or amplitude modulators, which are coupled to receive the first signal form a transponder antenna. Each signal conditioning element provides an intermediate signal having a known delay and a known amplitude modification to the acoustic wave interacting with it. Even where the signal is split into multiple portions, it is advantageous to reradiate the signal through a signal antenna. Therefore, a single "signal combining element" coupled to the all of the acoustic waves, which have interacted with the signal conditioning elements, is provided for combining the intermediate signals to produce the radiated transponder signal. The radiated signal is thus a complex composite of all of the signal modifications, which may occur within the transponder, modulated on the interrogation wave.
In known passive acoustic transponder system, the transponder remains static over time, so that the encoded information is retrieved by a single interrogation cycle, representing the state of the tag, or more typically, obtained as an inherent temporal signature of an emitted signal due to internal time delays. In order to determine a transfer function of a passive transponder device, the interrogation cycle may include measurements of excitation of the transponder at a number of different frequencies. This technique allows a frequency domain analysis, rather than a time domain analysis of an impulse response of the transponder.
Typically, the interrogator transmits a fist signal having a first frequency that successively assumes a plurality of frequency values within a prescribed frequency range. This first frequency may, for example, be in the range of 905-925 MHz, referred to herein as the nominal 915 MHz band, a frequency band that is commonly available for such use. The response of the tag to excitation at any given frequency is distinguishable from the response at other frequencies, due to the comparative difference of the differing frequency and fixed time delays. Advantageously in a known system, the frequency changes over time, so that the received response of the tag, delayed due to the internal structures, is at a different frequency than the simultaneously emitted signal, thus reducing interference an facilitating a frequency domain analysis.
Preferably, the passive acoustic wave transponder tag includes at least one element having predetermined characteristics, which assist in synchronizing the receiver and allows for temperature compensation of the system. As the temperature changes, the piezoelectric substrate may expanded contract, altering the characteristic delays and other parameters of the tag. Variations in the transponder response due to changes in temperature thus result, in part, from the thermal expansion of the substrate material. Although propagation distances are small, an increase in temperature of only 20.degree. C. can produce an increase in propagation time by the period of one entire cycle at a transponder frequency of about 915 MHz; correspondingly, a change of about 1.degree. C. results in a relative phase change of about 18.degree.. The acoustic wave is often a surface acoustic wave, although acoustic wave devices operating with various wave types are known.
The transponder is constructed such that i.sup.th delay time t.sub.i =T.sub.0 +K.DELTA.T+.DELTA.V.sub.i, where K is a proportionality constant, .DELTA.T is the nominal, known difference in delay time between the intermediate signals of two particular successive ones of the signal delay elements in the group, and .DELTA.V.sub.i is a modification factor due to inter-transponder variations, such as manufacturing variations. By measuring the quantities .DELTA.T and .DELTA.V.sub.i, it is possible, according to known techniques, to determine the expected delay time t.sub.i -T.sub.0 for each and every signal delay element from the known quantities K, .DELTA.T and .DELTA.V.sub.i. The manufacturing variations .DELTA.V.sub.i are comprised of a "mask" variation .DELTA.M.sub.i due to imperfections in the photolithographic mask; an "offset" variation .DELTA.O.sub.i which arises from the manufacturing process used to deposit the metal layer on the piezoelectric substrate; and a random variation .DELTA.R.sub.i which is completely unpredictable but usually neglectably small. Specific techniques are available for determining and compensating both the mask variations .DELTA.M.sub.i and the offset variations .DELTA.O.sub.i.
This known chirp interrogation surface acoustic wave transponder system provides a number of advantages, including high signal-to-noise performance, and the fact that the output of the signal mixer-namely, the signal which contains the difference frequencies of the interrogating chirp signal and the transponder reply signal--may be transmitted over inexpensive, shielded, twisted-pair wires because these frequencies are, for example, typically in the audio range, for passive transducer systems. Furthermore, since the audio signal is not greatly attenuated or dispersed when transmitted over long distances, the signal processor may be located at a position quite remote from the signal mixer, or provided as a central processing site for multiple interrogator antennae.
Passive transponder encoding schemes include selective modification of interrogation signal transfer function H(s) and delay functions f(z). These functions therefore typically generate a return signal in the same band as the interrogation signal. Since the return signal is mixed with the interrogation signal, the difference between the two will generally define the information signal, along with possible interference and noise. By controlling the rate of change of the interrogation signal frequency with response to a maximum round trip propagation delay, including internal delay, as well as possible Doppler shift, the maximum bandwidth of the demodulated signal may be controlled. Thus, the known systems employ a chirp interrogation waveform which allows a relatively simple processing of limited bandwidth transponder signals.
Known impulse excitation systems require broadband transponder signal analysis, and thus cannot typically employ audio frequency analysis systems.
The term "tap" refers either to the physical point of connection on, or to the instant of time that energy is removed from a "tapped" delay line. In the case of a surface acoustic wave (SAW) device, the term "tap" refers to the mechanism for, or instant of time that acoustic energy in a piezoelectric substrate is reconverted into electrical energy.
A known surface acoustic wave passive interrogator label system, as described, for example, in U.S. Pat. Nos. 4,734,698; 4,737,790; 4,703,327; and 4,951,057, includes an interrogator comprising a voltage controlled oscillator 10 which produces a fist signal S1 at a radio frequency determined by a control voltage V supplied by a control unit 12. This signal S1 is amplified by a power amplifier 14 and applied to an antenna 16 for transmission to a transponder 20. The voltage controlled oscillator 10 may be replaced with other oscillator types.
In one known system, the signal S1 is received at the antenna 18 of the transponder 20 and passed to a signal transforming element 22. This signal transformer converts the first (interrogation) signal S1 into a second (replay) signal S2, encoded with an information pattern. The information pattern is encoded as a series of elements having characteristic delay periods T.sub.0 and .DELTA.T.sub.1, .DELTA.T.sub.2, . . . .DELTA.T.sub.N. Two common types of systems exist. In a first, the delay periods correspond to physical delays in the propagation of the acoustic signal. After passing each successive delay, a portion of the signal I.sub.0, I.sub.1, I.sub.2 . . . I.sub.N is tapped off and supplied to a summing element. The resulting signal S2, which is the sum of the intermediate signals I.sub.0 . . . I.sub.N, is fed back to a transponder tag antenna, which may be the same or different than the antenna which received the interrogation signal, for transmission to the interrogator/receiver antenna. In a second system, the delay periods correspond to the positions of reflective elements, which reflect portions of the acoustic wave back to the launch transducer, where they are converted back to an electrical signal and emitted by the transponder tag antenna. The signal S2 is passed either to the same antenna 18 or to a different antenna 24 for transmission back to the interrogator/receiver apparatus. This second signal S2 carries encoded information which, at a minimum, identifies the particular transponder 20.
The signal S2 is picked up by a receiving antenna 26. Both this second signal S2 and the first signal S1 (or respective signals derived from these two signals) are applied to a mixer (four quadrant multiplier) 30 to produce a third signal S3 containing frequencies which include both the sums and the differences of the frequencies contained in the signals S1 and S2. The signal S3 is passed to a signal process 32 which determines the amplitude a.sub.i and the respective phase .phi..sub.i of each frequency component .phi..sub.i among a set of frequency components (.phi..sub.0, .phi..sub.1, .phi..sub.2 . . . ) in the signal S3. Each phase .phi..sub.i is determined with respect to the phase .phi..sub.0 =0 of the lowest frequency component .phi..sub.0. The signal S3 may be intermittently supplied to the mixer by means of a switch, and indeed the signal processor may be time-division multiplexed to handle a plurality of S3 signals from different antennas.
The information determined by the signal processor 32 is passed to a computer system comprising, among other elements, a random access memory (RAM) 34 and a microprocessor 36. This computer system analyzes the frequency, amplitude and phase information and makes decisions based upon this information. For example, the computer system may determine the identification number of the interrogated transponder 20. This I.D. number and/or other decoded information is made available at an output 38.
The transponder serves as a signal transforming element 22, which comprises N+1 signal conditioning elements 40 and a signal combining element 42. The signal conditioning elements 40 are selectively provided to impart a different response code for different transponders, and which may involve separate intermediate signals I.sub.0, I.sub.1 . . . I.sub.N within the transponder. Each signal conditioning element 40 comprises a known delay T.sub.i and a known amplitude modification A.sub.i (either attenuation or amplification). The respective delay T.sub.i and amplitude modification A.sub.i may be functions of the frequency of the received signal S1, or they may provide a constant delay and constant amplitude modification, respectively, independent of frequency. The time delay and amplitude modification may also have differing dependency on frequency. The order of the delay and amplitude modification elements may be reversed; that is, the amplitude modification elements A.sub.i may precede the delay elements T.sub.i. Amplitude modification A.sub.i can also occur within the path T.sub.i. The signals are combined in combining element 42 which combines these intermediate signals (e.g., by addition, multiplication or the like) to form the reply signal S2 and the combined signal emitted by the antenna 18.
In one known interrogation system embodiment, the voltage controlled oscillator 10 is controlled to produce a sinusoidal RF signal with a frequency that is swept in 128 equal discrete steps from 905 MHz to 925 MHz. Each frequency step is maintained for a period of 125 microseconds so that the entire frequency sweep is carried out in 16 milliseconds. Thereafter, the frequency is dropped back to 905 MHz in a relaxation period of 0.67 milliseconds. The stepwise frequency sweep 46 shown in FIG. 3B thus approximates the linear sweep 44 shown in FIG. 3A.
Assuming that the stepwise frequency sweep 44 approximates an average, linear frequency sweep or "chirp" 47, FIG. 3B illustrates how the transponder 20, with its known, discrete time delays T.sub.0, T.sub.1 . . . T.sub.N produces the second (replay) signal 52 with distinct differences in frequency from the first (interrogation) signal 51. Assuming a round-trip, radiation transmission time of t.sub.0, the total round-trip times between the moment of transmission of the first signal and the moments of reply of the second signal will be t.sub.0 +T.sub.0, t.sub.0 +T.sub.1, . . . t.sub.0 +T.sub.N, for the delays T.sub.0N, T . . . , T.sub.1 respectively. Considering only the transponder delay T.sub.N, at the time t.sub.R when the second (reply) signal is received at the antenna 26, the frequency 48 of this second signal will be .DELTA.f.sub.N less than the instantaneous frequency 47 of the first signal S1 transmitted by the antenna 16. Thus, if the first and second signals are mixed or "homodyned", this frequency difference .DELTA.f.sub.N will appear in the third signal as a beat frequency. Understandably, other beat frequencies will also result from the other delayed frequency spectra 49 resulting from the time delays T.sub.0, T.sub.1 . . . T.sub.N-1. Thus, in the case of a "chirp" waveform, the difference between the emitted and received waveform will generally be constant.
In mathematical terms, we assume that the phase of a transmitted interrogation signal is .phi.=2 .pi.f.tau., where .tau. is the round-trip transmission time delay. For a ramped frequency df/dt or f, we have: 2 .pi.f.tau.=d.phi./dt=.omega.. .omega., the beat frequency, is thus determined by .tau. for a given ramped frequency or chirp f. In this case, the signal S3 may be analyzed by determining a frequency content of the S3 signal, for example by applying it to sixteen bandpass filters, each turned to a different frequency, f.sub.0, f.sub.1 . . . f.sub.E, f.sub.F. The signal processor determines the amplitude and phase of the signals that pass through these respective filters. These amplitudes and phases contain the code or "signature" of the particular signal transformer 22 of the interrogated transponder 20. This signature may be analyzed and decoded is known manner.
In one embodiment of a passive transponder, shown in FIGS. 6 and 7, the internal circuit operates to convert the received signal S1 into an acoustic wave and then to reconvert the acoustic energy back into an electrical signal S2 for transmission via a dipole antenna 70, connected, and arranged adjacent a SAW device made of a substrate 72. More particularly, the signal transforming element of the transponder includes a substrate 72 of piezoelectric material such as a lithium niobate (LiNbO.sub.3) crystal, which has a free surface acoustic wave propagation velocity of about 3488 meters/second. On the surface of this substrate is deposited a layer of metal, such as aluminum, forming a pattern which includes transducers and delay/reflective elements.
One transducer embodiment includes a pattern consisting of two bus bars 74 and 76 connected to the dipole antenna 70, a "launch" transducer 78 and a plurality of "tap" transducers 80. The bars 74 and 76 thus define a path of travel 82 for a surface acoustic wave which is generated by the launch transducer and propagates substantially linearly, reaching the tap transducers each in turn. The tap transducers convert the surface acoustic wave back into electrical energy which is collected and therefore summed by the bus bars 74 and 76. This electrical energy then activates the dipole antenna 70 and is converted into electromagnetic radiation for transmission as the signal S2.
The tap transducers 80 are provided at equally spaced intervals along the surface acoustic wave path 82, as shown in FIG. 6, and an informational code associated with the transponder is imparted by providing a selected number of "delay pads" 84 between the tap transducers. These delay pads, which are shown in detail in FIG. 7, are preferably made of the same material as, and deposited with, the bus bars 74, 76 and the transducers 78, 80. Each delay pad has a width sufficient to delay the propagation of the surface acoustic wave from one tap transducer 80 to the next by one quarter cycle or 90.degree. with respect to an undelayed wave at the frequency of operation (in the 915 MHz band). By providing locations for three delay pads between successive tap transducers, the phase f of the surface acoustic wave received by a tap transducer may be controlled to provide four phase possibilities, zero pads=0.degree.; one pad=90.degree.; two pads=180.degree.; and three pads=270.degree.. The phase information .phi..sub.0 (the phase of the signal picked up by the first tap transducer in line), and .phi..sub.1, .phi..sub.2 . . . .phi..sub.N (the phases of the signals picked up by the successive tap transducers) is supplied to the combiner (summer) which, for example, comprises the bus bars 74 and 76. This phase information, which is transmitted as the signal S2 by the antenna 70, contains the informational code of the transponder.
As shown in FIG. 7, the three delay pads 84 between two tap transducers 80 are each of such a width L as to each provide a phase delay of 90.degree. in the propagation of an acoustic wave from one tap transducer to the next as compared to the phase in the absence of such a delay pad. This width L is dependent upon the material of both the substrate and the delay pad itself as well as upon the thickness of the delay pad and the wavelength of the surface acoustic wave.
While a system of the type described above operates satisfactorily when the number of tap transducers does not exceed eight, the signal to noise ratio in the transponder reply signal is severely degraded as the number of tap transducer increases. This is because the tap transducers additionally act as launch transducers as well as partial reflectors of the surface acoustic wave so that an increase in the number of tap transducers results in a corresponding increase in spurious signals in the transponder replies. This limitation on the number of tap transducers places a limitation on the length of the informational code imparted in the transponder replies.
Spurious signals as well as insertion losses may be reduced in a passive transponder so that the informational code may be increased in size to any desired length, by providing one or more surface acoustic wave reflectors on the piezoelectric substrate in the path of travel of the surface acoustic wave, to reflect the acoustic waves back toward a transducer for reconversion into an electric signal.
A transducer 86 may thus be employed in conjunction with reflectors 88 and 90 in a unique configuration which replaces the aforementioned arrangement having a launch transducer 78 and tap transducers 80. In particular, the transducer 86 is constructed to convert electrical energy received at the terminals 92 and 94 into surface acoustic wave energy which propagates outward in opposite directions indicated by the arrows 96 and 98. The launch transducer is constructed in a well known manner with an inter-digital electrode assembly formed of individual electrode fingers arranged between and connected to the two bus bars 100 and 102. In the illustrated pattern, half the fingers are connected to the bus bar 100 and the other half are connected to the bus bar 102. Each electrode is connected to one or the other bus bar and extends toward a free end in the direction of the other bus bar. The distance between the centers of successive fingers is equal to 3.lambda./4 where .lambda. is the center wavelength of the surface acoustic wave. Furthermore, as may be seen, the length of the active region between the ends of the electrodes connected to the bus bar 100 and the ends of the electrodes connected to the bus bar 102 is K.lambda., where K is a proportionality constant. Surface acoustic waves which travel outward from the transducer 86 in the directions 96 and 98 encounter and are reflected back by the reflectors 88 and 90. These reflectors comprise individual electrode fingers which extend between the bus bars 104 ad 106 on opposite sides. These electrodes are spaced from center to center, a distance .lambda./2 apart. The reflectors 88 and 90 serve to reflect nearly 100% of the surface acoustic wave energy back toward the transducer 86; that is, in the directions 108 and 110, respectively. Thus, after a pulse of surface acoustic wave energy is generated by the transducer 86, it is reflected back by the reflectors 88 and 90 and reconverted into an electrical signal by the transducer 86.
The configuration may also include one or more delay pads 112 which control the phase of the surface acoustic wave received back by the transducer 86. For a 90.degree. phase delay (as compared to the phase of the received surface acoustic wave without a delay pad present) the delay pad should have a width equal to 1/2 the width of the typical delay pads because the surface acoustic wave will traverse the delay pads twice (i.e., in both directions).
A plurality of transducers 114 may be connected to common bus bars 116 and 118 which, in turn, are connected to the dipole antenna of the transponder. On opposite sides of this configuration and reflectors 120 and 122 which reflect surface acoustic waves back toward the transducers which launched them. Since the transducers 114 are connected in parallel, a radio frequency interrogation pulse is received by all the transducers essentially simultaneously. Consequently, these transducers simultaneously generate surface acoustic waves which are transmitted outward in both directions. Due to the particular configuration shown, the reflected surface acoustic waves are received at staggered intervals so that a single interrogation pulse produces a series of reply pulses after respective periods of delay.
Another embodiment of a passive transponder includes four transducers 124 which are connected electrically in series between bus bars 126. These transducers are interconnected by means of intermediate electrodes 128, the electrical circuit through each transducer being effected by capacitive coupling. When energized by an RF electrical signal, the transducers simultaneously produce surface acoustic waves which travel in four parallel paths 130.
To the right of the transducers 124 are four sets 132, 134, 136 and 138 of reflectors 140 arranged in the paths of travel 130 of the surface acoustic waves. In the example shown, three reflectors 140 are arranged in each set; however, the number of reflectors may be varied. If only a single reflector is provided in each of the positions 132, 134, 136 and 138, this reflector should be designed to reflect nearly 100% of the surface acoustic waves at the wavelength of these waves. If more than one reflector is provided, these reflectors should be designed to reflect only a portion of the acoustic wave energy. Where three reflectors are provided in each set, the first and second reflectors should allow some of the acoustic wave energy to pass beneath them to the third and last reflector in line. In this way, if a pulse of surface acoustic wave energy is generated by a transducer 124, some of it will be reflected by the first transducer, some by the second and some by the third reflector in line.
Another transponder system provides separate launch and receiving transducers. As may be seen, surface acoustic waves are generated by a launch transducer 166 and propagated in the direction indicated by the arrow 168. These surface acoustic waves pass beneath the receiving transducer 170 and continue on toward one or more reflectors 172 in the direction indicated by the arrow 174. This acoustic wave energy is reflected by the reflectors 172 and directed back toward the receiving transducer 170 in the direction indicated by the arrow 176. The launch and receiving transducers may be connected to separate dipole antennas. This may be advantageous in certain applications since the different antennas may receive and radiate energy in different directions, and this allows separate signal processing for received and transmitted RF energy.
FIG. 8, a single launch transducer (LT) 90 transmits surface acoustic waves in both directions to tap transducers (T) 92, 94, 96 and 98. As may be seen, the launch transducer 90 is slightly offset (to the left as illustrated in FIG. 8) so that the length of the transmission path 1 to the tap transducer 92 is shorter than the path 2 to the tap transducer 94. Similarly, the path 3 to the tap transducer 96 is shorter than the path 4 too the tap transducer 98. In particular, the various transducers are positioned such that the differences in propagation times between the pathways 1 and 2, 2 and 3, and 3 and 4 are all equal (.DELTA.T). The outputs of the tap transducers 92, 94, 96 and 98 may thus be summed to produce a second signal S2 of the type represented in FIG. 5.
FIG. 9 illustrates the same basic configuration as in FIG. 8 except that the launch transducer 100 operates also to reconvert the SAW energy into electrical energy to form the signal S2. Reflectors 102, 104, 106 and 108 serve to reflect the acoustic wave energy proceeding on paths 1, 2, 3 and 4, respectively, back toward the transducer 100. As in the configuration of FIG. 8, the differences in propagation times between successive pathways (i.e., between pathways 1 and 2, 2 and 3, and 3 and 4) are all equal (.DELTA.T).
In the embodiments of FIG. 8 and FIG. 9, surface acoustic waves traveling along pathways 3 and 4 must pass beneath transducers 92, 94 (FIG. 8) or reflectors 102, 104 (FIG. 9). Such an arrangement of successive, multiple tap transducers or reflectors in a pathway introduces unwanted reflections and spurious signals into the output signal S2, making subsequent signal processing more difficult. FIGS. 10 and 11 illustrate SAW device configurations, corresponding to FIGS. 8 and 9, respectively, in which plural launch transducers simultaneously receive and convert the signal S1 into SAW energy. With this arrangement the pathways, 1, 2, 3 and 4 are spatially separated so that the surface acoustic waves can travel on the surface of the substrate without passing beneath a reflector or transducer. Combinations in various ways of the configurations of FIGS. 8-11 are also known. FIG. 12 shows an embodiment which combines the principles illustrated in FIGS. 9 and 11. In this embodiment, two launch/receive transducers 110 and 112 simultaneously convert the interrogation signal S1 into surface acoustic waves which travel along pathways 1,2,3,4,5,6,7 and 8. The transducers 110 and 112 are positioned so that the propagation times along these pathways are staggered, from one pathway to the next, by a fixed amount .DELTA.T; that is, the propagation time along pathway 2 is .DELTA.T longer than along pathway 1, the propagation time along pathway 3 is .DELTA.T longer than along pathway 2, etc. It will be appreciated that an information code can be imparted to the second (reply) signal S2 by means of "delay pads" of the type illustrated in FIGS. 6 and 7. These delay pads may be inserted at appropriate places along the respective propagation pathways illustrated in FIGS. 8-12.
The embodiment of FIG. 13 comprises a substrate 220 of piezoelectric material, such as lithium niobate, on which is deposited a pattern of metallization essentially as shown. The metallization includes two bus bars 222 and 224 for the transmission of electrical energy to four launch transducers 226, 228, 230 and 232. These launch transducers are staggered, with respect to each other, with their leading edges separated by distances X, Y and Z, respectively, as shown. The distances X and Z are identical; however, the distance Y is larger than X and Z in order to provide temporal separation of the received signals corresponding to the respective signal paths. Further metallization includes four parallel rows of delay pads 234, 236, 238 and 240 and four parallel rows of reflectors 242, 244, 246 and 248. The two rows of reflectors 244 and 246 which are closest to the transducers are called the "front rows" whereas the more distant rows 242 and 248 are called the "back rows" of the transponder. The bus bars 222 and 224 include contact pads 250 and 252, respectively, to which are connected the associated poles 254 and 256 of a dipole antenna. These two poles are connected to the contact pads by contact elements or wires 258 and 260, represented in dashed lines.
The embodiment of FIG. 13 is similar, in principle, to the embodiment of FIG. 12. The provision of four transducers 226, 228, 230 and 232 and two rows of reflectors 242, 244, 246, and 248 on each side of the transducers results in a total of sixteen SAW pathways of different length and, therefore, sixteen "taps". These sixteen pathways (taps) are numbered 0, 1, 2 . . . D, E, F, as indicated by the reference number (letter) associated with the individual reflectors. Thus, pathway 0 extends from transducer 226 to reflector 0 and back again to transducer 226 as shown in FIG. 9. Pathway 1 extends from transducer 228 to reflector 1 and back again to transducer 228. The spatial difference in length between pathway 0 and pathway 1 is twice the distance X (the offset distance between transducers 226 and 228). This results in a temporal difference of .DELTA.T in the propagation time of surface acoustic waves. Similarly, pathway 2 extends from transducer 226 to reflector 2 and back again to transducer 226. Pathway 3 extends from transducer 228 to reflector 3 and back to transducer 228. The distance X is chosen such that the temporal differences in the length of the pathway 2 with respect to that of pathway 1, and the length of the pathway 3 with respect to that of pathway 2 are also both equal to .DELTA.T. The remaining pathways 4, 5, 6, 7 . . . E, D, F are defined by the distances from the respective transducers launching the surface acoustic waves to the associated reflectors and back again. The distance Y is equal to substantially three times the distance X so that the differences in propagation times between pathway 3 and pathway 4 on one side of the device and pathway B and pathway C on the opposite side are both equal to .DELTA.T. With one exception, all of the temporal differences, from one pathway to the next successive pathway are equal to the same .DELTA.T. The SAW device is dimensioned so that .DELTA.T nominally equals 100 nanoseconds. In order to avoid the possibility that multiple back and forth propagations along a shorter pathway (one of the pathways on the left side of the SAW device as seen in FIG. 13) appear as a single back and forth propagation along a longer pathway (on the right side of the device), the difference in propagation times along pathways 7 and 8 is made nominally equal to 150 nanoseconds.
FIG. 15 is a graph illustrating the ranges of amplitudes which are expected in the individual components of the second (reply) signal associated with the respective pathways or tap delays 0-F. As may be seen, the greatest signal amplitudes will be received from pathways having reflectors in their front rows; namely, pathways 0,1,4,5,8,9,C and D. the signals received from the pathways having reflectors in their back rows are somewhat attenuated due to reflections and interference by the front row reflectors. If any one of the amplitudes a.sub.i at one of the sixteen frequencies f.sub.i in the third signal falls outside its prescribed range, the decoded identification number for that transponder is rejected.
As indicated above, transponders of the type illustrated in FIGS. 6-13 are susceptible to so-called "manufacturing" variations in response, due to manufacturing differences from transponder to transponder, as well as temperature variations in response due to variations in ambient temperature. Particularly the case where small differences in tap delays in the order of one SAW cycle period are measured to determine the encoded transponder identification number, these manufacturing and/or temperature variations can be in the order of magnitude of the informational signal.
As explained above, the transponder identification number contained in the second (reply) signal is determined by the presence or absence of delay pads in the respective SAW pathways. These delay pads make a slight adjustment to the propagation time in each pathway, thereby determining the phase of the surface acoustic wave at the instant of its reconversion into electrical energy at the end of its pathway. Accordingly, a fixed code (phase) is imparted to at least two pathways in the SAW device, and the propagation times for these pathways are used as a standard for the propagation times of all other pathways. Likewise, in a reflector-based acoustic device, a reflector may be provided at a predetermined location to produce a reference signal.
The mask variation .DELTA.M.sub.i for a given pathway, i.e., a variation in tap delay due to imperfections in the mask--will be the same for all transponders made from the same mask (typically, the mask for forming the transducers, reflectors and phase pads). The time variations .DELTA.O.sub.i is the so-called "offset" variation which is primarily due to variations in the interdigital finger line widths of a reflector in the front row through which the surface acoustic waves must pass to reach a reflector i in the back row. Variations in transducer finger line widths are already reflected in the initial pathway propagation time T.sub.0. These variations are traceable to the manufacturing process (such as the mask exposure time) and are normally the same for all parallel front row reflectors on one side of a transponder substrate. The line widths may vary from one side of the substrate to the other due to lack of orthogonality in the mask exposure. Since the time variations .DELTA.R.sub.i are completely random from pathway to pathway and from transponder to transponder, it is not possible to compensate for these. If a random variation .DELTA.R.sub.i becomes too large, however, the transponder identification number reading will be rejected, since one of the amplitudes a.sub.i or phases .phi..sub.i will fall outside of the acceptable limits. In addition, variations due to temperature which are reflected in large changes in the propagation times T.sub.0 and .DELTA.T must also be compensated. These temperature variations are substantially (but not exactly) the same for each pathway.
The three types of variations identified above-namely, temperature, mask and offset variations, are compensated in known systems as follows: Temperature variations are compensated by determining the times T.sub.0 and .DELTA.T from two successive pathways i and j to provide a first temperature estimate, and then compensating small, second order variations by averaging the propagation times of the four front row pathway pairs (pathways 0 and 1, 4 and 5, 8 and 9 and C and D). The variation .DELTA.M, which relates to the mask, will be the same for all transponders made from the same mask. Consequently, this variation may be isolated and compensated for by determining the amplitudes a.sub.i and phases .phi..sub.i for a large number of transponders, and thereafter determining statistically the acceptable limits for these parameters. By way of illustration, the amplitudes a.sub.i from different transponders made from the same mask for each frequency f.sub.i may be plotted on a graph such as that shown in FIG. 15 to determine their statistical distribution. The acceptable limits of amplitude may then be determined for each frequency from this statistical distribution. FIG. 15 shows one such distribution curve 170 of amplitudes for the frequency 2.45 kHz (pathway F). Variations in the phases .phi..sub.i of different transponders traceable to the mask are compensated in a similar manner by adjusting the center phases (nominally 0.degree., 90.degree., 180.degree. and 270.degree.) and the phase tolerances (nominally +/-30.degree. about each center phase) for each "phase bin". After the initial compensation for mask variations .DELTA.M, all subsequent masks used to manufacture transponders may be adjusted sa as to match the imperfections in the original mask. The mask variations .DELTA.M are therefore caused to remain identical for all transponders used in a given system. Finally, offset variations .DELTA.O, which are traceable to manufacturing process variations, are compensated by determining .DELTA.O.sub.F and using this value as a standard to eliminate the effect of offset in all the "back row" pathways; i.e., pathways 2,3,6,7,A,B and E.
The entire process of compensation is illustrated in the flow chart of FIG. 16. As is indicated there, the first step is to calculate the amplitude a.sub.i and phase .phi..sub.i for each audio frequency .phi..sub.i (block 180). Thereafter, the sixteen amplitudes are compared against their acceptable limits (block 182). As shown in FIG. 15, these limits may be different for each amplitude. If one or more amplitudes fall outside the acceptable limits, the transponder reading is immediately rejected. If the amplitudes are acceptable, the phase differences .phi..sub.ij are calculated (block 184) and the temperature compensation calculation is performed to determine the best value for .DELTA.T (block 186). Thereafter, the offset compensation calculation is performed (block 188) and the phases for the pathways 2,3,6,7,A,B and E are adjusted. Finally, an attempt is made to place each of the pre-encoded phases into one of the four phase bins (block 190). If all such phases fall within a bin, the transponder identification number is determined; if not, the transponder reading is rejected.